The thin film semiconductor solar cell of copper-indium-gallium-selenide (Cu,In,Ga,Se) having the chalcopyrite structure has been studied in recent years as is shown in some patent applications such as WO94/24696, WO96/06454. The structure of the solar cell is shown in FIG. 3 in which on a glass substrate 9, a bottom side electrode layer 2 of molybdenum (Mo), a photoelectric semiconductor layer of Cu,In,Ga,Se 1, a buffer layer of CdS 4 and a top side electrode layer 3 are formed sequentially. The top side electrode layer 3 is made by forming transparent electrode layers of ZnO, ZnO/Al 3a,3b on the buffer layer 4 sequentially, and by forming an Al electrode 3c comprising a plurality of separated strips 3c1, 3c2 of non-transparent conductive layers.
The molybdenum is selected as a material to form the bottom side electrode layer 2, because the Cu(In,Ga)Se2 photoelectric semiconductor layer 1 of the good quality can be formed on it. The soda-lime silica glass is one of suitable materials for the glass substrate 9. One reason why it is suitable for the glass substrate is that it has almost an equal thermal expansion coefficient to that of the Cu(In,Ga)Se2 semiconductor layer 1 which result in the reduction of the thermal strain in the stacked composite structure to improve the reliability of the solar cell.
The strips 3c1, 3c2 of the non-transparent conductive layers of the top side electrode 3c are extended in the direction diagonal to the cross section keeping an appropriate predetermined distance between each other. The width of each of the strips of the electrode layers 3c should be made as large as possible to decrease the resistance of the electrode. However, their width should be made as small as possible to minimize the amount of the solar light which will be reflected by them. Accordingly, their width, more precisely, the ratio of their width to their distance, should be determined to be the optimum value considering both factors.
The conventional solar cell shown in FIG. 3 has a problem that the Joule loss becomes large in the electrode layer 2, because the material molybdenum has a high electrical resistance. As a result, the efficiency of photoelectric energy conversion of the solar cell will be decreased.
In the conventional solar cell shown in FIG. 3, so called scribing method has been applied to make the electrical connection to the bottom side electrode layer 2 at it's peripheral area. In the scribing method, the top side electrode layer 3 and the semiconductor photoelectric conversion layer 1 at the peripheral area of the bottom side electrode layer 2 will be removed by scratching and breaking the overlying layers 1,3 using a sharp metal claw to expose the underlying electrode layer 2. However, this scribing method is difficult because only the very thin underlying electrode layer 2 must be left exposed without suffering substantial damage, which results in decrease of the yield of the fabrication.
To solve the problem, the present inventors had an idea to form a structure shown in FIG. 4. In the structure, an additional electrode 6 is formed on the opposite side of the glass substrate 5, and to which the bottom side electrode 2 is connected by conductive paths 9 formed in through holes made in the glass substrate 5. However, a new problem will arise that the quality of the semiconductor photoelectric layer of Cu(In,Ga)Se2 will be degraded locally just above the conductive paths 9 to cause a reduction of the overall conversion efficiency of the solar cell.